1. Field of the Invention
This invention relates to a drive system for a motor vehicle comprising means for permanently driving a first pair of wheel and automatically operable means for selectively driving a second pair of wheels in dependence on the slip of the first pair of wheels.
2. Description of the Prior Art
It is known that the slip of two permanently driven wheels of a motor vehicle can be determined in that their speed is compared with the speed of the wheels which can selectively be driven and that the latter wheels may be driven too when the slip of the permanently driven wheels exceeds a predetermined limit (FISITA Belgrade, June 2 to 6, 1986, Proceedings, Volume 2, pages 277 to 283). During a four-wheel drive operation the selectively driven wheels must mechanically be disconnected from the power train in shortly succeeding intervals of time to permit a renewed comparison of the slip of the permanently driven wheels and of the selectively drivable wheels so that it can be ascertained whether the slip of the permanently driven wheels is still so high that the four-wheel drive operation is called for. Without such a mechanical disconnection of the selectively driven wheels from the power train it would not be possible to detect a speed difference between the permanently driven wheels and the selectively driven wheels unless the means for driving the selectively drivable wheels include a differential or a liquid friction coupling. A clutch for the intermittent connection and disconnection of the selectively driven wheels to and from the power train in short intervals of time will be required to perform a cyclical operation under high load, and a differential which may be employed will increase the structural expenditure to an extent which may not be justified in some cases.